The waiting is the hard part. Many publishers do not want you to submit to anyone else until they have made their decision, so you can't just keep sending it out to all and sundry. I send it out and try to forget about it. Operative word is "try". I continue working with my critique group, I continue writing my next manuscript and pretend not to hold my breath. It's the best I can do. Then one day and envelope appears in the mail or an e-mail appears with the publishers address and you release that breath you have been holding for weeks or sometimes, months.
More wallpaper. I tease that I will someday paper my office in rejection letters as a backdrop for all my framed "New York Times Best Seller" titles. Someone told me Stephen King did that, so I guess I'll be in good company.
Rejection hurts, no matter how many times it happens to a writer. Sometimes editors take the time to tell you exactly why they turned you down, sometimes they say things that leave you guessing, most times, you get a form letter. It's all part of the process of becoming a writer.
I am not discouraged. I have a lot of qualified people who tell me my writing is good, funny, touching, my characters are wonderful. It's just a matter of getting my work to the right set of eyes.